Wednesday, November 13, 2013

New paper finds CFCs combined with other factors have had little effect on total atmospheric ozone over past century

A paper published today in Geophysical Research Letters finds tropospheric ozone has only changed a "very small" amount [about 0.8%] from pre-industrial times to the present when all of the combined effects of ozone depleting substances such as chlorofluorocarbons, N2O, ozone precursors, and climate change are considered together. The authors find "significant decreases in total column ozone over large parts of the Southern Hemisphere during austral spring and widespread increases in column ozone over the Northern Hemisphere during boreal summer."

This calls into question the role of man-made chlorofluorocarbons [CFCs] as the alleged cause of the Antarctic ozone hole and necessity or effectiveness of the Montreal Protocol. Since CFCs are well mixed in the atmosphere at approximately the same levels in the troposphere of the Northern and Southern Hemispheres, why would they have opposite effects in the Southern and Northern Hemipheres?

We investigate ozone changes from pre-industrial times to the present using a chemistry-climate model. The influence of changes in physical climate, ozone depleting substances [e.g. chloroflurocarbons], N2O and tropospheric ozone precursors is estimated using equilibrium simulations with these different factors set at either pre-industrial or present-day values. When these effects are combined, the entire decrease in total column ozone from pre-industrial to present-day is very small (−1.8 DU) in the global annual average, though with significant decreases in total column ozone over large parts of the Southern Hemisphere during austral spring and widespread increases in column ozone over the Northern Hemisphere during boreal summer. A significant contribution to the total ozone column change is the increase in lower-stratospheric ozone associated with the increase in ozone precursors (5.9 DU). Also noteworthy is the near cancellation of the global average climate change effect on ozone (3.5 DU) by the increase in N2O (−3.9 DU).

"For example, 300 DU of ozone brought down to the surface of the Earth at 0 °C would occupy a layer only 3 mm thick. One DU is 2.69×1016 ozone molecules per square centimetre, or 2.69×1020 per square metre. This is 0.4462 millimoles of ozone per square metre."Related: